1. Field of the Invention
The present invention relates to a gradation control system and a gradation control method for controlling printing or the like with a plurality of gradations and, in particular, relates to a gradation control system and a gradation control method for use in a thermal printer or the like.
2. Description of the Related Art
In recent years, for example, receipts issued by small thermal printers connected to POS (Point Of Sales) terminals etc. often include indication of messages etc. about advertisement of commodities and sales promotion, in addition to list of particulars about usual sales registration results. Accordingly, there has been a tendency that when printing the receipts, two-color printing using two-color thermosensitive paper or multi-color printing using the inkjet printing technique is carried out, i.e. the power of expression is required to the receipts themselves. On the other hand, there has been a problem that customers"" requirements about running cost, such as cost of the two-color paper as compared with general thermosensitive paper and cost of inks, are severe so that the spread of two-colored or multi-colored receipts is not liable to be advanced.
Incidentally, as prior art literature relating to the present invention, there have been the following:
(1) JP-A-H11-115234
(2) JP-A-H04-220358
Therefore, by realizing multi-gradation printing using thermal printers that utilize the conventional general thermosensitive paper, it has been implemented to give the power of expression to receipts in the manner of, for example, advertisement of commodities with photographs or stamp printing.
Conventionally, in a thermal printer that performs a multiple gradation printing by superimposing heat energy per dot to utilize a coloring characteristic of thermosensitive paper, it is necessary to apply the same dot a number of times according to a gradation degree as shown, for example, in FIG. 3 of the patent document (1). In case of, for example, printing n-gradation data, heat energy is superimposed by repeatedly carrying out an applying operation nxe2x88x921 times at maximum. In this case, there is a problem that as the number of gradations increases, the number of times of transfer to a thermal head also increases. More specifically, since a time required for performing an applying operation with respect to one dot line is prolonged in proportion to the number of gradations, an operation of conveying a paper to the next line is kept waiting. That is, there has been a problem that as the number of gradations increases, the printing speed is obliged to be decreased.
On the other hand, as described in the patent document (2), there is a method that transfers a binary gradation value expression as it is to a thermal head as energization data and gives a bit weight to an energizing time itself. In this method, as opposed to the foregoing method, when performing the n-gradation application, it is sufficient to perform transfer to the thermal head by the number of bits that express it. However, there is a problem that because of physical restriction of a time for transferring data to the thermal head, a difference in energizing time per one gradation is restricted by the time for transferring data. More specifically, when explanation is given using a time chart of the conventional method shown in FIG. 8, assuming that a time required for serial transfer of data to the thermal head is, for example, 32 xcexcsec, an interval from latching of certain data to latching of the next data is required to be 32 xcexcsec or more. It means that, referring to FIG. 8, while latching data PT0, it is necessary to transfer the next data PT1, and thus, if a time for this data transfer is 32 xcexcsec, a time from latching of PT0 to latching of PT1 is required to be 32 xcexcsec or longer. Namely, this means that an energizing time for the data PT0 is required to be at least 32 xcexcsec in this case, and this problem is actualized particularly in the following case. FIG. 2 shows a heat energy vs. coloring concentration characteristic of general thermosensitive paper. Setting of the minimum gradation difference is restricted due to, for example, the following reason, so that, as shown in FIG. 2, if a gradation control is executed with a time T8 per gradation difference, a coloring concentration difference per gradation becomes large in a half-tone coloring region that is the most important and thus should be fractionized when carrying out the gradation printing. Therefore, there has been a problem that these concentration differences work as one factor that impedes the smooth gradation printing.
Needless to say, a similar problem exists in the patent document 1.
Therefore, it is an object of the present invention to solve such a conventional problem, and provide a gradation control system and method that can determine a target minimum energizing time and minimum gradation difference value without depending on a physical transfer time of a thermal head.
According to a first aspect of the present invention, there is provided a gradation control system comprising means for decoding lower n bits (n is an integer equal to 1 or greater than 1) of a binary number expressing a gradation per dot, to derive 2n decoded data; means for grouping the 2n decoded data with respect to a plurality of dots, to produce 2n first bit planes; means for giving to an element a signal of pattern corresponding to each of the first bit planes for a time obtained by adding a time that depends on a decoded value of each of the first bit planes, to an initializing time common to all of the first bit planes; means for grouping, per bit weight, respective bits of the binary number excluding the lower n bits, that expresses the gradation per dot, to produce at least one second bit plane; and means for giving to the element a signal of pattern corresponding to each second bit plane for a time that depends on a weight of the second bit plane.
According to a second aspect of the present invention, there is provided a gradation control system comprising means for decoding lower n bits (n is an integer equal to 1 or greater than 1) of a binary number expressing a gradation per dot, to derive 2n decoded data; means for grouping the 2n decoded data with respect to a plurality of dots, to produce 2n first bit planes; means for giving to an element a signal of pattern corresponding to each of the first bit planes for a time obtained by subtracting an unconditional time common to all of the first bit planes from a time obtained by adding a time that depends on a decoded value of each of the first bit planes, to an initializing time common to all of the first bit planes; means for giving to the element a signal of all bit effective pattern for the unconditional time; means for grouping, per bit weight, respective bits of the binary number excluding the lower n bits, that expresses the gradation per dot, to produce at least one second bit plane; and means for giving to the element a signal of pattern corresponding to each second bit plane for a time that depends on a weight of the second bit plane.
In the gradation control system according to the first or second aspect of the present invention, it is preferable that the initializing time is a time for causing a recording medium to reach a coloring region.
In the gradation control system according to the first or second aspect of the present invention, it is preferable that the initializing time is longer than a transfer time for each bit plane.
The gradation control system according to the first or second aspect of the present invention may further comprise means for giving to the element a signal corresponding to a history of use of each dot.
According to a third aspect of the present invention, there is provided a thermal printer comprising the gradation control system according to the first or second aspect of the present invention.
A first effect of the present invention resides in that, by fractionizing an energizing time with respect to a region where a concentration variation is the largest in a coloring characteristic of thermosensitive paper, so as to precisely control color development of a half-tone portion, smoother gradation printing can be realized.
The reason thereof is that, by adding an initial energizing time necessary for thermosensitive paper to start color development, to the minimum value of an energizing time so as to execute a control, the minimum energizing time that determines the minimum gradation difference is not restricted by, for example, a physical transfer time that is inevitably generated upon carrying out serial data transfer to a thermal head.
A second effect of the present invention resides in that a gradation control can flexibly deal with a coloring characteristic that changes depending on a kind of thermosensitive paper, a thermal head to be used, or an outside air temperature.
The reason thereof is that the factor that restricts the minimum gradation difference is eliminated as referred to in the reason of the first effect, and that since a value of binary data is checked to enable a gradation difference to be determined by software, variation of the coloring characteristic can be flexibly dealt with.